A nuclear reactor, such as a pressurized water reactor, comprises a core formed by prismatic arrays arranged side by side inside the reactor vessel and resting on the lower core-supporting plate, which is arranged transversely relative to the axis of the vessel.
Each of the arrays consists of a bundle of parallel fuel rods located between two ends and extending in the longitudinal direction of the array. The arrays rest with one of their ends, or lower end, on the lower core-supporting plate, so that the direction of the fuel rods, or axial direction of the array, is vertical. Each of the arrays is arranged on the lower core-supporting plate, in correspondence with an opening passing through this plate.
In its upper part, the core is limited by a horizontal plate parallel to the core-supporting plate, and also pierced with openings opposite each of the fuel arrays.
In this way, the reactor cooling fluid, for example pressurized water, which is fed in under the lower core plate, passes through the arrays from bottom to top, cooling them at the same time, and leaves at the upper part of the core to transfer the heat from the latter to the heat exchangers.
In the case of certain pressurized water reactors, the arrays have a hexagonal cross-section and are arranged next to one another in the reactor core, so as to form a uniform mesh in the transverse planes of the core. These arrays are not arranged continuously, so as to avoid lateral interference between the arrays, in the reactor in operation.
A clearance of the order of three millimeters is therefore maintained between the lateral faces of the hexagonal-based prisms consituting the outer envelope of the arrays. This clearance is maintained by means of centering elements, such as studs, arranged on the upper plate and on the core supporting base, cooperating with housings providing in the corresponding ends of the arrays and making it possible to keep the arrays in position laterally, relative to one another. A clearance must be provided between the centering studs and their housing, this clearance generally being of the order of one millimeter. These clearances make it possible to compensate the discrepancies in alignment between the centering studs of the upper core plate and the corresponding studs of the core-supporting base, and also to compensate the effects of the differential expansion between the upper core plate and the lower plate for supporting this core.
An axial clearance must also be provided in order to avoid interference in the axial direction, due to the differential expansion between the arrays and the upper plate and the lower core-supporting plate, respectively.
The cooling fluid passing through the core at high speed, from bottom to top, exerts a vertical force on the arrays, which tends to displace them upwards. Impacts between the arrays and the upper plate or the lower core-supporting plate, respectively, are thus capable of being produced during the starting or stopping of the primary pumps.
To overcome this disadvantage, holding devices are used which make it possible to prevent the impacts between the arrays and the upper and lower core plates, while at the same time permitting relative axial displacements due to expansion.
For example, springs have been used which are arranged between the upper part of the array and the upper core plate and which exert, on these arrays vertical forces directed downwards, for holding the latter against the lower core-supporting plate. However, these springs exert, on the structure of the arrays, i.e., on the guide tubes joined to each of the ends or on the case of the array, compression forces which can result in buckling of the fuel array.
It has also been proposed to fasten the array to the lower core-supporting plate by means of elements projecting from the upper face of the core-supporting plate and firmly fixed to the latter. These projecting elements are arranged in the region of the corners of the array and possess locking surfaces which cooperate with a locking element carried by the array.
However, a fixing device of this type is bulky and of complex construction and makes the positioning of the arrays in the core more difficult.